Homebrew, Ravenloft: Denizens of Darkness (3.5e)
Keen Hearing and Smell. The fenhound has advantage on Wisdom (Perception) checks that rely on hearing or smell.
Magical Immunities. The fenhound is immune to any damage or the effects of spells cast by a cleric in the service of a deity of the sun, the moon, moors, or revenge. It is also immune to any damage or the effects of spells cast by a cleric of the Light Domain subclass.
Pack Tactics. The fenhound has advantage on an attack roll against a creature if at least one of the wolf's allies is within 5' of the creature and the ally isn't incapacitated.
Rejuvenation. If the fenhound is killed, upon the next night of the full moon it will reappear in a new body on the same plane of existence and regains all its hit points. The creature responsible for killing the fenhound becomes its new target. An attonement using the Ceremony spell can free a creature targeted this way from the fenhound's attention.
Vengeful Tracker. The fenhound knows the distance to & direction of the creature it is targeting, even if the creature & the fenhound are on different planes of existence. If the creature being tracked by the fenhound dies, the revenant knows.
Multiattack. The fenhound makes two bite attacks.
Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8 + 4) piercing damage. If the target is a creature, it must succeed on a DC 14 Strength saving throw or be knocked prone.
Bay (Recharges 5-6). The fenhound howls to the moon, which can be heard for a mile in every direction. Every creature that can hear the howl must succeed on a DC 13 Charisma saving throw or become frightened for one minute. Any creature of a good alignment has advantage on the saving throw. The creature that is the fenhound's target has disadvantage on the saving throw.
Hills, Swamp